Stimulating electrodes were chronically implanted in the ventral hippocampal commissure and the entorhinal cortex or angular bundle of rats. Moveable metal microelectrodes which could be passed through the hippocampus were implanted. All hippocampal units were classified s complex-spike cells or theta cells on the basis of the form of their action potentials and their rates of firing in various behaviors. Field potentials and unit firing evoked from the stimulating electrodes were recorded during slow wave sleep. Complex-spike cells (1) could often be antidromically activated in CA3 (it was not attempted in CA1); (2) could only be induced to fire one or two action potentials in response to a single stimulus; (3) had action potentials at the same time as the local population-spike and, in condition-test studies, were depressed when the population-spike was depressed. (The population-spike is presumably the summed synchronous action potentials of pyramidal cells.) Theta cells: (1) were antidromically activated in only one out of 25 cases; (2) usually could fire long bursts of action potentials in response to a sufficiently intense single stimulus; (3) this firing occurred before, during, and after the local orthodromic population-spike. Most complex-spike cells in Ammon's horn must be pyramidal cells (projection cells), and vice versa. The case for theta cells is more difficult. Some are non-pyramidal cells with locally ramifying axons, but at least some are projection cells. The data is consistent with most of them being inhibitory interneurons, but this is not established.